Photonic crystals are composite materials with periodically arranged domains, such that the periodic modification of the dielectric function leads to constructive interference of the reflected light for certain wavelengths. The regions of the spectrum that are completely reflected are referred to as photonic bandgaps. The energy of allowed modes of propagation as a function of wavevector is generally referred to as the “bandstructure” of the photonic crystal. The bandgap is where there are no modes for light of a specific energy to propagate through the material. The bandstructure of a photonic crystal can be different for each polarization of light (e.g., right-handed circular polarization or left-handed circular polarization), depending on the symmetry of the material.
Ruthenium complexes may be used to mediate the ring-opening metathesis polymerization of sterically bulky macromonomers (MMs) to high molecular weight (MW) molecular brush copolymers. Exploiting the advantageous characteristics (i.e. stability, livingness, functional group and steric tolerance) of catalysts such as 1 (FIG. 2), well-defined, high MW brush copolymers with quantitative grafting density have been synthesized. More so, the living nature of this polymerization system has enabled sequential and statistical copolymerization of various MM's. In the case of brush block copolymers, microphase segregation to extremely large polymer domains can be rapidly achieved, producing photonic band-gap materials that can reflect long wavelengths of light. Additionally, if the grafts are composed of rigid polyisocyanates, self-assembly to even larger domains is facilitated, and infrared-reflecting (IR-reflecting) materials can be fabricated through simple controlled evaporation.
Living polymerization of isocyanates may be accomplished by using halftitanocene complexes as initiators, which quantitatively incorporate a specific chain-end group. As an example, this approach is an efficient means to produce exo-norbornene functionalized MM's, which have been shown to be excellent substrates for ROMP. Furthermore, polyisocyanates are a class of polymers known to assume a helical conformation with excess one-handed helicity in the presence of a chiral bias. Optically active helical polyisocyanate grafts are able to influence the chiroptical properties of brush copolymers. As an example, chirality of polyisocyanate grafts may be transferred to a low degree-of-polymerization (DP) (DP˜20) polyphenylacetylene main-chain, thereby inducing excess one-handed helicity into the main-chain. Separately, tethering one-handed helical polyisocyanate grafts to a polystyrene main-chain induces a switch in the helix-sense of the polyisocyanate grafts once the brush polymer aggregates.
The difficulty of incorporating chiral elements into photonic crystals has limited the ability to generate unique bandstructures for different circular polarizations of light. As such, there is a need to develop methods of easily, predictably fabricating chiral photonic crystals having these properties. Chiral photonic crystals have a number of applications in optics.